Effective magnetic permeability

Effective magnetic permeability (also apparent magnetic permeability1)), often denoted as μe, μeff or μa - a term used in analysis of magnetic performance of gapped cores. For a non-homogeneous core (e.g. gapped or composed of powder-like particles) this would be the value of magnetic permeability of a hypothetical homogeneous material which would exhibit the same permeability.
2)

With the gap present, higher magnetomotive force (excitation) is required to reach the same flux density. Similar behaviour could be obtained if the magnetic circuit was made not from a gapped core but from a non-gapped core made from material with proportionally lower permeability. A value of permeability required to obtain equivalent B-H performance is therefore the value of effective permeability.

Note: The equation is valid only for a simple magnetic circuit, made out of bulk material, for relative permeability if lcore >> lgap, and if µcore >> 1.

The equation is derived by using the concept of magnetic reluctance6) and with the assumptions listed above. All values of permeability (input and output) are given as relative permeability (so the value of “1” means permeability of the air gap itself). The length of the core and the gap must be given in the same units. For instance, if the core length is given in millimetres, then also the air gap length must be given in millimetres. But the equation holds for any other length units: inches/inches, metres/metres, etc.

Equations can also be derived for multi-path or non-uniform magnetic circuits, but these are obviously configuration-dependent and must be calculated for each specific structure.7)

Composite materials

The ratio of air gap and the particles in a powder core dictate the value of effective permeability. The black lines illustrate distribution of magnetic flux.

The value of effective permeability is important for composite materials, which may contain significant volumetric percentage of non-magnetic material. The small particles (as in powder cores) have rather high permeability, but the bulk of the core made out of such material exhibits effective permeability whose value is tailored for specific applications.8)

However, because such a magnetic core does not have a concentrated air gap then the simple equation given above cannot be used. Depending on the complexity of given material the calculations can become very difficult to solve or even formulate.9)

Hence, the end users of composite cores can rely on the effective permeability values given by the manufacturers of the materials or magnetic cores. If the product is a magnetic core, then the AL value (inductance per turn) is often more useful than the value effective permeability as such.10)

Nevertheless, in order to easier distinguish the type of material from which a given core is made the name of the material often refers to the value of effective permeability, for instance Ferroxcube uses notation Sendust 75, where 75 is the value of effective relative permeability at room temperature, so that11)μe = 75.0